The trend in disk file design is constantly towards higher track densities allowing a greater amount of information to be recorded on a single disk. In a common type of disk file, a number of disks are arranged in a stack with spacers in between each disk. This stack of disks is then supported on a surface e.g. a shelf extending radially from the hub for rotation about an axis passing through the spindle of the disk motor. An axial force is applied to clamp the disk stack in position. In such disk drives, it is important to ensure that once information has been written to each of the disks or to a dedicated servo disk, there is minimal or no shift of the disks out of concentricity with the axis of rotation.
In disk files where the disks and the portion of the hub supporting the disks are made of materials having different coefficients of thermal expansion, the different expansions during thermal cycling cause large forces to be generated at the hub to disk interface. When the thermal expansion forces become large, the friction force generated by the clamp load is insufficient to restrain the expansion forces and the disks can shift radially. This movement is usually unpredictable in magnitude and direction and can cause the disks to move off the axis of rotation. The resultant track eccentricity may lead to data already on the disk becoming irretrievable.
There have been several attempts described in the art directed to solving this problem.
GB 2 163 525 describes a washer made from MYLAR (registered trade mark) located between the lowermost disk and the hub flange (the flange and the disc having different coefficients of thermal expansion). Changes in temperature result in relative movement between the disc and the flange which results in shearing of the washer. The washer acts (by resilience) to restore the disk to its original position when the temperature returns to its original value however this has been found not to be repeatable in practice.
U.S. Pat. No. 4,754,351 describes a different solution wherein axial strips define a clearance between the spindle and the disc and act so as to maintain a substantially uniform clearance upon thermally induced differential expansion of the disk.
U.S. Pat. No. 4,717,977 describes the use of an aluminum sleeve located around the steel motor cylinder, the engagement between the two parts being limited to one section near the centre of the overlapping surfaces. In this case, motor size is restricted by the need for the additional aluminum sleeve.
PCT application WO 88/06781 describes an in-hub motor assembly for a disk drive wherein the disks are mounted on a hollow aluminum spindle which contains a removable steel sleeve within which the motors magnets are mounted. Because the spindle is aluminum, thermal distortion caused by differential thermal expansion is eliminated but again, motor size is restricted.